The present invention relates to a method and arrangement for increasing the dynamic range at an input stage of a receiver in an optical fibre information transmission system.
An optical receiver is usually designed to be as sensitive as possible. The best results have been obtained by means of a high-impedance input stage. However, dynamic properties of such realizations are limited, and when strong optical signals are received the input stage is saturated and the detected signal is distorted. A kind of compromise solution is a transimpedance input stage. In commercially available PIN-FET modules, for instance, the sensitivities of the transimpedance input stages are even several decibels lower than those of high-impedance input stages; on the other hand, their dynamic range is larger.
Several special solutions have been developed for increasing the dynamic range. In U.S. Pat. Nos. 4,473,745 and 4,498,197, e.g., the problem has been tried to be solved by means of an RF-PIN diode which shunts part of the signal current generated by a photodiode o the earth when the received optical power is high. On the other hand, U.S. Pat. No. 4,540,952, for instance, utilizes for the same purpose a field effect transistor serving as an adjustable resistance.
The signal current generated by a photodiode includes both a dc component and an ac component, i.e. a proper signal component. If the signal is symmetric (an NRZ signal), the value I.sub.d of the dc component is in the same range as the peak value I.sub.p of the ac component, as illustrated in FIG. 1. In the U.S. Pat. Nos. 4,473,745 and 4,498,197, a detected dc component is applied directly to an RF-PIN diode attenuator as a control current, which is convenient, because the resistance of the RF-PIN diode is inversely proportional to the controlling current at high frequencies. As compared to the controlling dc component, the ac component, however, is so large that the diode functions within a non-linear range. Therefore it has been necessary in U.S. Pat. Nos. 4,473,745 and 4,498,197 to use an RF-PIN diode in which the carrier lifetime is so long that not even the lowest frequency components of the signal spectrum are distorted.
An insufficient carrier lifetime of RF-PIN diodes gives rise to problems at lower speeds, especially if signals having a poor balance or containing long "0" or "1" sequences are to be received. As a matter of fact, the control current of the attenuator disappears or is doubled with "0" and "1" sequences respectively. This results in heavy distortion of the signal.
The bandwidth of a high-impedance input stage of a receiver varies with the resistance of the attenuation diode. Therefore an adjustable RC equalizer has to be provided at a output of a amplifier of the input stage for compensating variation in the bandwidth. In U.S. Pat. No. 4,498,197, an RF-PIN diode is utilized in the equalizer as a varying resistance, and the RF-PIN diode is adjusted by means of the same control current as the attenuation diode. The resistance of these two RF-PIN diodes is thereby always the same. However, it is necessary for maintaining a proper signal-to-noise ratio, with low transmission speeds in particular, that a capacitance of a capacitor of said equalizer is considerably higher than a capacitance at the input of the amplifier. As a result, a resistive element of the equalizer has to be correspondingly smaller than the resistance at the input of the amplifier in order that the equalizer would function properly. Consequently, the realization according to U.S. Pat. No. 4,498,107, in which a common control current is used, often causes problems in practice. The first amplification stage always amplifies the signal component. This leads to another problem because the ac component has been increased at the equalizer diode, whereas the control current has remained unchanged. This further aggravates the nonlinearity problems.